int main(int argc, char* const argv[])
{
    //聲明AppRuntime類的實例runtime,在AppRuntime類的構(gòu)造方法中初始化的skia圖形引擎
    AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
    ...
    bool zygote = false;
    bool startSystemServer = false;
    bool application = false;
    String8 niceName;
    String8 className;
    ++i;  // Skip unused "parent dir" argument.
    while (i < argc) {
        const char* arg = argv[i++];
        if (strcmp(arg, "--zygote") == 0) {
            zygote = true;
            //對于64位系統(tǒng)nice_name為zygote64; 32位系統(tǒng)為zygote
            niceName = ZYGOTE_NICE_NAME;
        } else if (strcmp(arg, "--start-system-server") == 0) {
            //是否需要啟動system server
            startSystemServer = true;
        } else if (strcmp(arg, "--application") == 0) {
            //啟動進(jìn)入獨立的程序模式
            application = true;
        } else if (strncmp(arg, "--nice-name=", 12) == 0) {
               //niceName 為當(dāng)前進(jìn)程別名，區(qū)別abi型號
            niceName.setTo(arg + 12);
        } else if (strncmp(arg, "--", 2) != 0) {
            className.setTo(arg);
            break;
        } else {
            --i;
            break;
        }
    }
    ...
}

-Xzygote /system/bin --zygote --start-system-server --socket-name=zygote

Vector<String8> args;
    if (!className.isEmpty()) {
        // We're not in zygote mode, the only argument we need to pass
        // to RuntimeInit is the application argument.
        //
        // The Remainder of args get passed to startup class main(). Make
        // copies of them before we overwrite them with the process name.
        args.add(application ? String8("application") : String8("tool"));
        runtime.setClassNameAndArgs(className, argc - i, argv + i);
        if (!LOG_NDEBUG) {
          String8 restOfArgs;
          char* const* argv_new = argv + i;
          int argc_new = argc - i;
          for (int k = 0; k < argc_new; ++k) {
            restOfArgs.append(""");
            restOfArgs.append(argv_new[k]);
            restOfArgs.append("" ");
          }
          ALOGV("Class name = %s, args = %s", className.string(), restOfArgs.string());
        }
    } else {
        // We're in zygote mode.
        //初始化Dalvik虛擬機(jī)Cache目錄和權(quán)限
        maybeCreateDalvikCache();
        if (startSystemServer) {
           //附加上start-system-serve 的arg
            args.add(String8("start-system-serve 的argr"));
        }
        char prop[PROP_VALUE_MAX];
        if (property_get(ABI_LIST_PROPERTY, prop, NULL) == 0) {
            LOG_ALWAYS_FATAL("app_process: Unable to determine ABI list from property %s.",
                ABI_LIST_PROPERTY);
            return 11;
        }
        String8 abiFlag("--abi-list=");
        abiFlag.append(prop);
        args.add(abiFlag);
        // In zygote mode, pass all remaining arguments to the zygote
        // main() method.
        for (; i < argc; ++i) {
            args.add(String8(argv[i]));
        }
    }
    if (!niceName.isEmpty()) {
        runtime.setArgv0(niceName.string(), true /* setProcName */);
    }
    if (zygote) {
        //進(jìn)入此分支
        runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
    } else if (className) {
        runtime.start("com.android.internal.os.RuntimeInit", args, zygote);
    } else {
        fprintf(stderr, "Error: no class name or --zygote supplied.\n");
        app_usage();
        LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
    }

void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
     ...
    // 虛擬機(jī)創(chuàng)建及啟動，主要是關(guān)于虛擬機(jī)參數(shù)的設(shè)置
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    if (startVm(&mJavaVM, &env, zygote) != 0) {
        return;
    }
    onVmCreated(env);
    //注冊JNI方法
    if (startReg(env) < 0) {
        ALOGE("Unable to register all android natives\n");
        return;
    }
    /*
     * We want to call main() with a String array with arguments in it.
     * At present we have two arguments, the class name and an option string.
     * Create an array to hold them.
     */
    jclass stringClass;
    jobjectArray strArray;
    jstring classNameStr;
    //等價于strArray[0] = "com.android.internal.os.ZygoteInit"
    stringClass = env->FindClass("java/lang/String");
    assert(stringClass != NULL);
    strArray = env->NewObjectArray(options.size() + 1, stringClass, NULL);
    assert(strArray != NULL);
    classNameStr = env->NewStringUTF(className);
    assert(classNameStr != NULL);
    env->SetObjectArrayElement(strArray, 0, classNameStr);
      //strArray[1] = "start-system-server"；
      //strArray[2] = "--abi-list=xxx"；
      //其中xxx為系統(tǒng)響應(yīng)的cpu架構(gòu)類型，比如arm64-v8a.
    for (size_t i = 0; i < options.size(); ++i) {
        jstring optionsStr = env->NewStringUTF(options.itemAt(i).string());
        assert(optionsStr != NULL);
        env->SetObjectArrayElement(strArray, i + 1, optionsStr);
    }
    /*
     * Start VM.  This thread becomes the main thread of the VM, and will
     * not return until the VM exits.
     */
     //將"com.android.internal.os.ZygoteInit"轉(zhuǎn)換為"com/android/internal/os/ZygoteInit
    char* slashClassName = toSlashClassName(className != NULL ? className : "");
    jclass startClass = env->FindClass(slashClassName);
    if (startClass == NULL) {
        ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
        /* keep going */
    } else {
        //找到這個類后就繼續(xù)找成員函數(shù)main方法的Mehtod ID
        jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
            "([Ljava/lang/String;)V");
        if (startMeth == NULL) {
            ALOGE("JavaVM unable to find main() in '%s'\n", className);
            /* keep going */
        } else {
            // 通過Jni調(diào)用ZygoteInit.main()方法
            env->CallStaticVoidMethod(startClass, startMeth, strArray);
#if 0
            if (env->ExceptionCheck())
                threadExitUncaughtException(env);
#endif
        }
    }
    free(slashClassName);
    ALOGD("Shutting down VM\n");
    if (mJavaVM->DetachCurrentThread() != JNI_OK)
        ALOGW("Warning: unable to detach main thread\n");
    if (mJavaVM->DestroyJavaVM() != 0)
        ALOGW("Warning: VM did not shut down cleanly\n");
}

public static void main(String argv[]) {
        //ZygoteServer 是Zygote進(jìn)程的Socket通訊服務(wù)端的管理類
        ZygoteServer zygoteServer = null;
        // 標(biāo)記zygote啟動開始，調(diào)用ZygoteHooks的Jni方法，確保當(dāng)前沒有其它線程在運(yùn)行
        ZygoteHooks.startZygoteNoThreadCreation();
       //設(shè)置pid為0，Zygote進(jìn)入自己的進(jìn)程組
        try {
            Os.setpgid(0, 0);
        } catch (ErrnoException ex) {
            throw new RuntimeException("Failed to setpgid(0,0)", ex);
        }
        Runnable caller;
        try {
            ...
            //開啟DDMS(Dalvik Debug Monitor Service)功能
            RuntimeInit.enableDdms();
            //解析app_main.cpp - start()傳入的參數(shù)
            boolean startSystemServer = false;
            String zygoteSocketName = "zygote";
            String abiList = null;
            boolean enableLazyPreload = false;
            for (int i = 1; i < argv.length; i++) {
                if ("start-system-server".equals(argv[i])) {
                    //啟動zygote時，傳入了參數(shù)：start-system-server,會進(jìn)入此分支
                    startSystemServer = true;
                } else if ("--enable-lazy-preload".equals(argv[i])) {
                    //啟動zygote_secondary時，才會傳入?yún)?shù)：enable-lazy-preload
                    enableLazyPreload = true;
                } else if (argv[i].startsWith(ABI_LIST_ARG)) {
                    abiList = argv[i].substring(ABI_LIST_ARG.length());
                } else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
                    //SOCKET_NAME_ARG 為 zygote 或zygote_secondary，具體請參考 init.zyoget64_32.rc文件
                    zygoteSocketName = argv[i].substring(SOCKET_NAME_ARG.length());
                } else {
                    throw new RuntimeException("Unknown command line argument: " + argv[i]);
                }
            }
            // 根據(jù)傳入socket name來決定是創(chuàng)建socket還是zygote_secondary
            final boolean isPrimaryZygote = zygoteSocketName.equals(Zygote.PRIMARY_SOCKET_NAME);
            if (abiList == null) {
                throw new RuntimeException("No ABI list supplied.");
            }
            // In some configurations, we avoid preloading resources and classes eagerly.
            // In such cases, we will preload things prior to our first fork.
            // 在第一次zygote啟動時，enableLazyPreload為false，執(zhí)行preload
            if (!enableLazyPreload) {
                bootTimingsTraceLog.traceBegin("ZygotePreload");
                EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
                        SystemClock.uptimeMillis());
                // 加載進(jìn)程的資源和類
                preload(bootTimingsTraceLog);
                EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
                        SystemClock.uptimeMillis());
                bootTimingsTraceLog.traceEnd(); // ZygotePreload
            } else {
                Zygote.resetNicePriority();
            }
            // Do an initial gc to clean up after startup
            bootTimingsTraceLog.traceBegin("PostZygoteInitGC");
            gcAndFinalize();
            bootTimingsTraceLog.traceEnd(); // PostZygoteInitGC
            bootTimingsTraceLog.traceEnd(); // ZygoteInit
            // Disable tracing so that forked processes do not inherit stale tracing tags from
            // Zygote.
            Trace.setTracingEnabled(false, 0);
            Zygote.initNativeState(isPrimaryZygote);
            ZygoteHooks.stopZygoteNoThreadCreation();
            //  調(diào)用ZygoteServer 構(gòu)造函數(shù)，創(chuàng)建socket Server端，會根據(jù)傳入的參數(shù)，
             // 創(chuàng)建兩個socket：/dev/socket/zygote 和 /dev/socket/zygote_secondary
            zygoteServer = new ZygoteServer(isPrimaryZygote);
            if (startSystemServer) {
                //fork出system server進(jìn)程
                Runnable r = forkSystemServer(abiList, zygoteSocketName, zygoteServer);
                // {@code r == null} in the parent (zygote) process, and {@code r != null} in the
                // child (system_server) process.
                if (r != null) {
                    // 啟動SystemServer
                    r.run();
                    return;
                }
            }
            Log.i(TAG, "Accepting command socket connections");
           // ZygoteServer進(jìn)入無限循環(huán)，處理請求
            caller = zygoteServer.runSelectLoop(abiList);
        } catch (Throwable ex) {
            Log.e(TAG, "System zygote died with exception", ex);
            throw ex;
        } finally {
            if (zygoteServer != null) {4
                zygoteServer.closeServerSocket();
            }
        }
        // We're in the child process and have exited the select loop. Proceed to execute the
        // command.
        if (caller != null) {
            caller.run();
        }
    }

static void preload(TimingsTraceLog bootTimingsTraceLog) {
        beginPreload();
        //預(yù)加載類
        preloadClasses();
        cacheNonBootClasspathClassLoaders();
        //加載圖片、顏色等資源文件
        preloadResources();
        //加載HAL相關(guān)內(nèi)容
        nativePreloadAppProcessHALs();
       //加載圖形驅(qū)動
        maybePreloadGraphicsDriver();
        // 加載 android、compiler_rt、jnigraphics等library
        preloadSharedLibraries();
        //用于初始化文字資源
        preloadTextResources();
        //用于初始化webview;
        WebViewFactory.prepareWebViewInZygote();
        endPreload();
        warmUpJcaProviders();
        sPreloadComplete = true;
    }

 private static void preloadClasses() {
        final VMRuntime runtime = VMRuntime.getRuntime();
        //preload classes 路徑為 /system/etc/preloaded-classes
        InputStream is;
        try {
            is = new FileInputStream(PRELOADED_CLASSES);
        } catch (FileNotFoundException e) {
            Log.e(TAG, "Couldn't find " + PRELOADED_CLASSES + ".");
            return;
        }
       ...
        try {
            BufferedReader br =
                    new BufferedReader(new InputStreamReader(is), Zygote.SOCKET_BUFFER_SIZE);
            int count = 0;
            String line;
            while ((line = br.readLine()) != null) {
                // Skip comments and blank lines.
                line = line.trim();
                if (line.startsWith("#") || line.equals("")) {
                    continue;
                }
                Trace.traceBegin(Trace.TRACE_TAG_DALVIK, line);
                try {
                   //使用Class.forName初始化類
                    Class.forName(line, true, null);
                    count++;
                } catch (ClassNotFoundException e) {
                    Log.w(TAG, "Class not found for preloading: " + line);
                } catch (UnsatisfiedLinkError e) {
                    Log.w(TAG, "Problem preloading " + line + ": " + e);
                } catch (Throwable t) {
                    Log.e(TAG, "Error preloading " + line + ".", t);
                    if (t instanceof Error) {
                        throw (Error) t;
                    }
                    if (t instanceof RuntimeException) {
                        throw (RuntimeException) t;
                    }
                    throw new RuntimeException(t);
                }
                Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
            }
        } catch (IOException e) {
            Log.e(TAG, "Error reading " + PRELOADED_CLASSES + ".", e);
        } finally {
           ...
        }
    }

private static void preloadResources() {
        final VMRuntime runtime = VMRuntime.getRuntime();
        try {
            mResources = Resources.getSystem();
            mResources.startPreloading();
            if (PRELOAD_RESOURCES) {
                Log.i(TAG, "Preloading resources...");
                long startTime = SystemClock.uptimeMillis();
                //裝載com.android.internal.R.array.preloaded_drawables中的圖片資源
                TypedArray ar = mResources.obtainTypedArray(
                        com.android.internal.R.array.preloaded_drawables);
                int N = preloadDrawables(ar);
                ar.recycle();
                Log.i(TAG, "...preloaded " + N + " resources in "
                        + (SystemClock.uptimeMillis() - startTime) + "ms.");
                startTime = SystemClock.uptimeMillis();
                //裝載com.android.internal.R.array.preloaded_color_state_lists中的顏色資源
                ar = mResources.obtainTypedArray(
                        com.android.internal.R.array.preloaded_color_state_lists);
                N = preloadColorStateLists(ar);
                ar.recycle();
                Log.i(TAG, "...preloaded " + N + " resources in "
                        + (SystemClock.uptimeMillis() - startTime) + "ms.");
                if (mResources.getBoolean(
                        com.android.internal.R.bool.config_freeformWindowManagement)) {
                    startTime = SystemClock.uptimeMillis();
                    //裝載com.android.internal.R.array.preloaded_freeform_multi_window_drawables中的圖片資源
                    ar = mResources.obtainTypedArray(
                            com.android.internal.R.array.preloaded_freeform_multi_window_drawables);
                    N = preloadDrawables(ar);
                    ar.recycle();
                    Log.i(TAG, "...preloaded " + N + " resource in "
                            + (SystemClock.uptimeMillis() - startTime) + "ms.");
                }
            }
            mResources.finishPreloading();
        } catch (RuntimeException e) {
            Log.w(TAG, "Failure preloading resources", e);
        }
    }

private static void preloadSharedLibraries() {
        Log.i(TAG, "Preloading shared libraries...");
        System.loadLibrary("android");
        System.loadLibrary("compiler_rt");
        System.loadLibrary("jnigraphics");
    }

 private static Runnable forkSystemServer(String abiList, String socketName,
            ZygoteServer zygoteServer) {
       ...
       //配置system server 
        String args[] = {
                "--setuid=1000",
                "--setgid=1000",
                "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1023,"
                        + "1024,1032,1065,3001,3002,3003,3006,3007,3009,3010",
                "--capabilities=" + capabilities + "," + capabilities,
                "--nice-name=system_server",
                "--runtime-args",
                "--target-sdk-version=" + VMRuntime.SDK_VERSION_CUR_DEVELOPMENT,
                "com.android.server.SystemServer",
        };
        ZygoteArguments parsedArgs = null;
        int pid;
        try {
            //將啟動參數(shù)封裝到ZygoteArguments類中
            parsedArgs = new ZygoteArguments(args);
            Zygote.applyDebuggerSystemProperty(parsedArgs);
            Zygote.applyInvokeWithSystemProperty(parsedArgs);
            boolean profileSystemServer = SystemProperties.getBoolean(
                    "dalvik.vm.profilesystemserver", false);
            if (profileSystemServer) {
                parsedArgs.mRuntimeFlags |= Zygote.PROFILE_SYSTEM_SERVER;
            }
            //fork systemserver子進(jìn)程，最終會進(jìn)入com_android_internal_os_Zygote.cpp 類中fork進(jìn)程并做一些初始化操作
            pid = Zygote.forkSystemServer(
                    parsedArgs.mUid, parsedArgs.mGid,
                    parsedArgs.mGids,
                    parsedArgs.mRuntimeFlags,
                    null,
                    parsedArgs.mPermittedCapabilities,
                    parsedArgs.mEffectiveCapabilities);
        } catch (IllegalArgumentException ex) {
            throw new RuntimeException(ex);
        }
        if (pid == 0) {
            //pid == 0 ，處理system server的邏輯
            if (hasSecondZygote(abiList)) {
                // 處理32_64和64_32的情況
                waitForSecondaryZygote(socketName);
            }
            // fork時會copy socket，system server需要主動關(guān)閉
            zygoteServer.closeServerSocket();
            // 裝載system server相關(guān)邏輯
            return handleSystemServerProcess(parsedArgs);
        }
        return null;
    }

private static Runnable handleSystemServerProcess(ZygoteArguments parsedArgs) {
        Os.umask(S_IRWXG | S_IRWXO);
        if (parsedArgs.mNiceName != null) {
            //nicename 為 system_server
            Process.setArgV0(parsedArgs.mNiceName);
        }
        ...
        if (parsedArgs.mInvokeWith != null) {
            String[] args = parsedArgs.mRemainingArgs;
            // If we have a non-null system server class path, we'll have to duplicate the
            // existing arguments and append the classpath to it. ART will handle the classpath
            // correctly when we exec a new process.
            if (systemServerClasspath != null) {
                String[] amendedArgs = new String[args.length + 2];
                amendedArgs[0] = "-cp";
                amendedArgs[1] = systemServerClasspath;
                System.arraycopy(args, 0, amendedArgs, 2, args.length);
                args = amendedArgs;
            }
            WrapperInit.execApplication(parsedArgs.mInvokeWith,
                    parsedArgs.mNiceName, parsedArgs.mTargetSdkVersion,
                    VMRuntime.getCurrentInstructionSet(), null, args);
            throw new IllegalStateException("Unexpected return from WrapperInit.execApplication");
        } else {
           //parsedArgs.mInvokeWith 為null,會進(jìn)入此分支
            createSystemServerClassLoader();
            ClassLoader cl = sCachedSystemServerClassLoader;
            if (cl != null) {
                Thread.currentThread().setContextClassLoader(cl);
            }
            /*
             * Pass the remaining arguments to SystemServer.
             */
            return ZygoteInit.zygoteInit(parsedArgs.mTargetSdkVersion,
                    parsedArgs.mRemainingArgs, cl);
        }
        /* should never reach here */
    }

public static final Runnable zygoteInit(int targetSdkVersion, String[] argv,
           ClassLoader classLoader) {
      ...
       RuntimeInit.commonInit();
       //注冊兩個jni函數(shù)
       //android_internal_os_ZygoteInit_nativePreloadAppProcessHALs
       //android_internal_os_ZygoteInit_nativePreloadGraphicsDriver
       ZygoteInit.nativeZygoteInit();
       return RuntimeInit.applicationInit(targetSdkVersion, argv, classLoader);
   }

protected static Runnable applicationInit(int targetSdkVersion, String[] argv,
            ClassLoader classLoader) {
         //true代表應(yīng)用程序退出時不調(diào)用AppRuntime.onExit()，否則會在退出前調(diào)用
        nativeSetExitWithoutCleanup(true);
       //設(shè)置虛擬機(jī)的內(nèi)存利用率參數(shù)值為0.75
        VMRuntime.getRuntime().setTargetHeapUtilization(0.75f);
        VMRuntime.getRuntime().setTargetSdkVersion(targetSdkVersion);
        final Arguments args = new Arguments(argv);
        // Remaining arguments are passed to the start class's static main
        return findStaticMain(args.startClass, args.startArgs, classLoader);
    }

 protected static Runnable findStaticMain(String className, String[] argv,
            ClassLoader classLoader) {
        Class<?> cl;
        try {
            //這里className為 com.android.server.SystemServer
            cl = Class.forName(className, true, classLoader);
        } catch (ClassNotFoundException ex) {
            throw new RuntimeException(
                    "Missing class when invoking static main " + className,
                    ex);
        }
        Method m;
        try {
            m = cl.getMethod("main", new Class[] { String[].class });
        } catch (NoSuchMethodException ex) {
            throw new RuntimeException(
                    "Missing static main on " + className, ex);
        } catch (SecurityException ex) {
            throw new RuntimeException(
                    "Problem getting static main on " + className, ex);
        }
        int modifiers = m.getModifiers();
        if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
            throw new RuntimeException(
                    "Main method is not public and static on " + className);
        }
        /*
         * This throw gets caught in ZygoteInit.main(), which responds
         * by invoking the exception's run() method. This arrangement
         * clears up all the stack frames that were required in setting
         * up the process.
         */
        return new MethodAndArgsCaller(m, argv);
    }